Meat itself is not a living organism but it is subject to endogenic enzymatic activity, or proteolysis, which causes muscle tissue to mature, become tender and develop a typical taste. This process is retarded by cold.
Due to its chemical composition which is rich in proteins, lipids and water, meat is a particularly favourable substrate for the growth of microorganisms. The lipidic content also makes it very sensitive to oxidation.
Healthy animals, hygienically slaughtered after resting and fasting, provide a practically aseptic meat. However, following slaughter the evisceration and dressing operations inevitably produce microbial contamination in depth and especially on the surface, through contact with equipment, tools, hands and clothes, despite all precautions.
Again, micro-organism growth is a temperature-dependent process. To avoid it, it is absolutely essential to reduce the temperature of the meat, especially on the surface, immediately after dressing. Cooling must therefore be carried out in the slaughterhouse itself. This operation is known as primary chilling.
Meat loses weight through surface evaporation. This process depends on differences in temperature and relative humidity between the meat and the environment.
Slaughter operations and carcass dressing separate the parts of the animal which have distinct histological properties and are intended for different uses. The carcass itself incorporates mainly muscles, bones, fat and connective tissue. The offal includes some edible organs, while some glands are used in pharmaceutical preparations. These different parts must be subjected to varying cooling conditions according to their susceptibility to microbial growth, to temperature effects and to the risk of surface dehydration.
To prevent or even to reduce the deterioration process, particularly microorganism development, chilling has to be carried out quickly after carcass dousing at the end of the slaughter process and the chilled state has to be maintained until the meat is processed for consumption.
Chilling can be defined as the fundamental operation in applying cold to meat to reduce its temperature quickly. This is done in a cold chamber with intensive air draught or movement. Rapid cooling of the meat surface not only slows and nearly stops the development of surface micro-organisms but also reduces weight loss and discoloration of the surface owing to haemoglobin oxidation. Different systems of primary chilling are in use (including immersion in iced water, especially for poultry) but air chilling is the most common.
The cold chambers where chilling takes place must have a low air temperature, a high air speed, a high relative humidity and a high refrigerating capacity.
Air temperature must be in the region of 0°C, with no decrease below -1°C, which could freeze the meat surface and impair its appearance.
Air speed can range from 0.25 to 3.0 m/s. However, for economical reasons the most common speeds in use are from 0.75 to 1.5 m/s in the empty section of the cold chamber.
Air speed over the carcasses will be much higher because of the reduction in air circulation. Increased air speed reduces the cooling period but it has a limit as there is a threshold above which fan-power consumption increases more than the chilling rate, resulting in an increase in operational costs. Also, the higher the air speed the greater the weight loss.
Relative humidity during the chilling operation should be kept fairly high to prevent excessive weight loss. The recommended rate is between 90 and 95 percent, though this is the most difficult factor to control.
Primary chilling is completed when the warmest point of the carcass has reached a temperature of about 7°C (3°C for edible offal). With current technology these temperatures can be arrived at in 16–24 hours in small carcasses and in less than 48 hours in large carcasses (centre of the hind leg). Average and surface temperatures are obviously much lower, reaching 0°C on the surface within four hours; this is important to slow microbial proliferation.
Quick chilling has its problems, cold shortening being the most common. Cold shortening can often be seen in beef and mutton, when the meat, still in its pre-rigor phase, reaches temperatures of 10°C or lower. These conditions cause irreversible contractions of the muscle tissue which toughen the meat even after prolonged ripening.
Quick primary chilling also signifies an increase in investment and higher operational costs. The chilling period can be reduced by lowering the air temperature (surface freezing risks) or increasing air speed (higher operational costs) or both. Occasionally cold chambers are refrigerated in advance to reach lower temperatures than those in operation (-5°C/-6°C for beef; -10°C/-12°C for pork), taking advantage of thermal inertia to offset the effect of warm meat loads.
Quick primary chilling can be performed in small chambers or in cooling tunnels. In cold chambers it is carried out in two or three phases. During the first phase the air temperature is maintained at about 0°C, carefully controlling the risk of superficial freezing while air movement is maintained at a high level. For large carcasses, after 10–12 hours the air circulation inside the store is reduced, maintaining temperature and humidity conditions; this second phase lasts another six to 10 hours. After this period the meat is transferred to cold storage chambers where the carcass temperature is stabilized, concluding the third phase.
Small cold chambers used for chilling must be designed so their capacity can be filled in two hours at the slaughterhouse's normal work rate. The number of chambers should be sufficient for a peak working day. Particular care should be taken that warm humid carcasses are placed behind those already chilled or in the process of being chilled so that the air, which is still cold, reaches them and there is no risk of superficial condensation.
Cooling tunnels used for chilling meat are usually of the continuous type. Here again meat is subjected to a two-phase process, with conditions similar to the cold chamber. However the temperature can be as low as -5°C for a short time. Beef carcasses can reach an average temperature of about 15°C in a four-hour period, while pork and mutton reach the same temperature in two to two and a half hours. Surface temperature decreases to 4–5 °C. During the second phase, conditions are less exacting, and an average temperature of about 4°C is stabilized after 15–16 hours in a secondary refrigerating chamber. This method is used in high-capacity slaughterhouses particularly for pig carcasses; for beef and mutton slower cooling is recommended because of the dangers of cold shortening.
Stored chilled meat is mainly intended to serve as buffer stock between production and shipment and/or consumption. During storage, ageing (ripening) of the meat also occurs, progressively increasing tenderness and developing taste through the proteolytic activity of meat enzymes. Ageing depends on temperature and can be accelerated by increasing it, but for hygienic reasons it is recommended that 4°C be used with a relative humidity of 85–95 percent. In these conditions ageing takes place in a few hours for poultry, two to four days for pork, four days for mutton and two weeks for beef. It can thus be considered as complementary treatment only for the last two products.
When chilled meat is stored for long periods a lower temperature without the risk of freezing should be used; normally 0°C is a reasonable choice, though (as shown in Table 1) conditions differ according to the type of meat.
A temperature of about 4°C is used in butcher shops (for final ageing, due to the difficulty of maintaining lower temperatures as the cold store rooms are small). Relative humidity is between 80 and 90 percent, which is a compromise between weight loss and microbial development; 80 percent is normally used for carcasses and quarters, and 85–90 percent for small meat cuts.
The preservation of edible offal requires different conditions: -1°C rather than 0°C and a relative humidity close to saturation to avoid surface blemishes. Organs intended for therapeutic purposes, such as thyroid, pancreas, ovaries, pituitary and so on, must be frozen immediately to preserve their active principles.
Table 1 gives the maximum storage time in which the products can be kept safe and keep their commercial quality during the subsequent marketing period, even if this is short and in favourable climatic conditions.
However, there is wastage and some loss of quality and nutritive value when a meat carcass is stored for the whole period indicated in Table 1. It is therefore recommended that the storage time should not exceed by much the ripening period required for the different types of meat.
Air circulation inside industrial chambers should be at a rate of 20–35 times per hour the volume of the empty cold room. When the chambers are used to store offal it is advisable to use natural air circulation to maintain high humidity levels.
Carcasses should be hung on rails in such a way that they are aligned in the direction of air circulation, avoiding contact with each other (see Figures 1 and 2).
Whenever a new product at a temperature different from that of the store is placed in store the product should be distributed around the room rather than concentrated in one place.
In many countries a deficient cold chain and insufficient cold storage capacity sometimes make it necessary to store different products in the same room. These products may be incompatible because they require different storage temperatures or they present some risk of tainting, through the transfer of aromas from one to another. The first type of risk is not a problem with animal products as they keep reasonably well under similar temperature conditions (obviously reference is made here only to chilled storage). Nevertheless the lowest recommended temperature without risk of superficial freezing should always be used.
Tainting is likely when meats or other animal products are stored with odorous fruits like oranges or apples, and the risk is more severe when there is mixed storage with potatoes. However, it is unusual to store meats with vegetables, so precautions are mainly needed when storing mixed animal products. There is some danger of cross action between beef and bacon; cheese will taint beef, mutton and pork.
Tainting must be guarded against not only during mixed storage but also when using a cold chamber which has previously stored produce with a strong tainting potential. Chambers must be thoroughly cleaned before any other product is stored.
Like the ageing of meat during cold storage, such as when butchers keep their stock at 4°C and 85–95 percent relative humidity, other complementary treatments are also used to lengthen the storage period, maintaining quality and reducing the risk of microbial spoilage.
A modified atmosphere is one of the treatments used nowadays for animal products, though not to such an extent as for fruit and vegetables. This technology employs a gas-composed atmosphere which is different from the normal (i.e. 21 percent O2, 79 percent N2 and minor contents of other gases).
A more common complementary treatment for meat storage is the vacuum packaging of boneless meat cuts. Special extremely airtight (oxygen-tight) synthetic films have been developed which can be heatsealed after removing the air around the packed meat cut, thus keeping it practically out of contact with the surrounding atmosphere. Provided hygienic slaughter and cutting methods are used, the shelf-life of meat packed in this way and stored under 0°/-1°C can be remarkably extended (up to eight weeks for beef, four weeks for lamb and two to three weeks for pork), which is important for the export of boneless chilled meat from meatproducing countries. This type of packaging is widely used for shipments of dried beef and mutton.
In special cases radiation is used as a complementary treatment to extend the shelf-life of chilled meat carcasses. However, this treatment is subject to national food legislation and is not allowed in many countries.
UV light (200–320 nm) is also used to reduce surface microbial contamination on meat and meat products. As the cuts have irregular shapes it is rather difficult to achieve the same radiation intensity, so it is normal procedure to irradiate the most contaminated zones. Radiation intensity produced by a 30W UV lamp is enough for every 10–12 m2 of floor space in a slaughterhouse or cold chamber.
Height of suspension and spacing for half carcasses on hooks
Height of suspension and spacing for quarters
Ionizing radiation is a promising complementary treatment for chilled meat preservation. Low doses of radiation are sufficient to reduce microbial contamination and the best prospects are for packaged meats which cannot be recontaminated. Ionizing radiation can also be employed to destroy trichinae (Trichinella spiralis) in pork meats.
Freezing is usually limited to meat to be used as buffer stock, frequently intended for export or for storage with a view to later processing.
When the preservation period is longer than that acceptable for chilled meat, freezing must be used to minimize any physical, biochemical and microbiological changes affecting quality in storage. During freezing most of the water content of the meat, about 80 percent, solidifies into pure ice crystals, accompanied by a separation of dissolved solids.
A product can be considered frozen when its centre has a temperature of -12°C or less. To reach this temperature the product passes through the temperature range of maximum crystallization (from -1° to -5°C). The speed of freezing is a very important factor as frozen meat quality depends mainly on the size of the ice crystal formed: the lower the speed of freezing the larger the size of the crystals.
Slow freezing facilitates the separation of solution and the migration of water out of the muscle cells which is subsequently frozen, forming rather large crystals. Quick freezing conversely produces many small ice crystals, mainly formed within the muscle cells, and reduces water migration and separation of solution. It is obvious that the latter technology will preserve the meat closer to its original quality and, particularly during thawing, moisture loss will generally be lower.
The International Institute of Refrigeration (IIR) expresses the freezing speed as the velocity with which a temperature front moves through the body of the product (cm/h). Good results are attained when the speed is from 2 to 5 cm/h. Slow freezing is considered to be below 1 cm/h and quick freezing above 5 cm/h.
Meat can be treated before freezing, generally being refrigerated to a chilled condition. Cutting into quarters is usual, particularly for large animals, and the fat is removed from some parts because though it prevents surface desiccation it reduces the heat transfer rate, and is susceptible to damage during frozen storage.
The relationship between thickness and freezing speed favours cutting and deboning before freezing, either as lean meat packaged in cardboard boxes or cut into individual portions. This has many advantages:
Freezing is performed in tunnels or in chambers with intense air circulation called blast chambers. Air temperatures should be in the range of -30° to -35°C; sometimes -40°C is used. Air is circulated at high speed, from 2 to 4 m/s and up to 6 m/s. An air circulation coefficient of 150–300 is used inside freezing chambers. Relative humidity is maintained at 95 percent or above.
In these conditions half beef carcasses or quarters are frozen in about 16– 20 hours, cut-up meat in cardboard boxes measuring 54×34×16 cm in about four hours and small prepackaged cuts in about one hour.
Small boxes and cuts, particularly of offal, are sometimes frozen in surface contact freezers (plate freezers): the product is pressed between two metallic plates cooled by direct expansion refrigerant. For items 3–5 cm thick, freezing time is as low as two to three hours.
After freezing carcasses and quarters must be protected with plastic film, usually under cloth or jute fabric. Meat cuts are covered with plastic film, or vacuum-packed in plastic bags; they are placed inside cardboard boxes and usually frozen in these.
When meat cuts are prepackaged without vacuum, air pockets must be avoided. A 2-cm space should be left in the upper part of the box to allow for expansion. Superficial fat should be eliminated before freezing to reduce the development of rancidity during storage.
TABLE 2. Practical storage life of meat and meat products
|Products||Practical storage life in months|
|-18 °C||-25 °C||-30 °C|
Roasts, steaks, packaged
Ground meat, packaged, (unsalted)
Bacon (green, unsmoked)
|Poultry, chicken and turkeys, eviscerated,|
From: Recommendations for the processing and handling of frozen foods, International Institute of Refrigeration, Paris, 1972.
Meats properly frozen are transferred from the freezer to storage chambers where temperature, relative humidity and air circulation should be adequate and can be tightly controlled. In particular fluctuations in temperature must be kept to a very narrow time interval.
As there is a certain degree of quality deterioration, even at very low temperatures, storage life is limited. The usual temperatures are in the range of -18° to -25°C for periods of preservation of one year or more. However, each type of meat requires specific conditions. Table 2 gives some approximate data regarding these. The higher the relative humidity the better: a range of 95–98 percent prevents meat dehydration.
For frozen meats and other animal products storage incompatibility is low. The temperature level needed in the chamber is similar for all of them, and tainting is negligible owing to the low temperature and to the fact that most of the products are in adequately protective packages.
The main problem with frozen storage is deterioration in organoleptic quality. There may be changes in meat texture, fat can become granular and crumble, and there can also be some discoloration of the meat. Fat modification induced by air oxygen produces rancidity and acidity, and a disagreeable taste. Microbial enzymes also remain active, especially those that attack the fat.
As in chilled storage, there are also weight losses through evaporation. This can be seen as freezer burn, i.e. superficial desiccated areas which can occur even in packaged meats when the packaging film is loose and temperature fluctuates inside the chamber. Weight loss, which can be between 1 and 4 percent in unpacked meat, favours organoleptic deterioration. The surface of the meat grows dry and porous, encouraging rancidity and transfer of aromas. In addition, the area of surface sublimation of frozen meat is very large: 12 m2/t for beef quarters, 11 m2/t for pork and 20 m2/t for mutton.
PSL is the storage period from the time of freezing for as long as the product maintains its organoleptic and nutritive characteristics and is suitable for human consumption or for further processing.
PSL relies on high quality raw material, good industrial practice, including hygiene, and the use of a reasonably constant temperature. PSL is therefore clearly dependent on PPP factors—product, processing and packaging.
Processing refers mainly to preliminary treatment and the freezing operation. Slaughtering, dressing of carcasses, preliminary chilling, cutting and deboning, and prepackaging of small cuts, must be conducted hygienically and by skilled labour.
In addition to personal hygiene, and cleaning and disinfection programmes in slaughterhouses, chilling facilities and cutting rooms, particular care should be taken when cutting and deboning and packaging, keeping contamination of the meat to a minimum. Carcasses should preferably be cut while hanging or on regularly cleaned surfaces, with tools frequently sterilized during operation and the meat stored in clean containers. The packaging material should be of good quality and clean.
Packaging is intended to preserve products from microbial contamination, from dehydration and from environmental factors that affect quality and nutrition. The materials used, besides being specifically for food, must be chemically inert and prevent the transfer of foreign odours or flavours. They must be stable at low and high temperatures, elastic, tear-resistant, and proof against water vapour, oxygen and volatile substances. They must offer protection against light, particularly UV light. Moreover they should be adaptable to different automatic packaging systems, of an appropriate size and shape for easy storing and distribution, and ready for opening.
Plastic films and papers and cardboard lined with plastic film are often used. For special packaging, different plastic films can be combined to take advantage of the main properties of each. Plastics of interest to the meat industry for cold storage are:
There is a daily quality loss in frozen meat storage that is cumulative, i.e. the total quality loss through freezing, storage, transport and distribution can be calculated by adding the losses at each step of the process. The tolerance of a product to a fixed temperature and time storage can be determined and expressed in figures. Figures or graphs representing practical storage life in different conditions can be established through time tolerance and tolerance (TTT) studies. These are run at at least three temperatures, around those normally used for storing the product under test (-18°, -25° and -30°C for instance) and show the quality relationship to timetemperature conditions.
As temperature fluctuations highly influence the final quality of the frozen product, its refrigeration history should be known. This together with the TTT characteristics of the product will allow the residual storage time to be calculated.
The rotation of stock throughout the cold chain should be organized according to the first in-first out (FIFO) rule: the first lots to be stored are the first to be unloaded.
Thawing is another critical phase in the freezing process as it involves a change from ice crystals to melted water, which is reabsorbed, and microbial reactivation.
If heat is applied to the frozen product its surface becomes warm enough to transfer heat to the inside and create conditions of temperature and humidity suitable for microbial development. Low temperature thawing, below 5°C, reduces the risk of microbial growth and produces a slow thawing rate which guarantees efficient reabsorption of the melted water.
It is recommended that carcasses be thawed at 4° to 6°C, in a hanging position and without any covering (plastic or jute is removed), inside a cold chamber with a reasonably low level of air circulation - about 0.2 m/s. Relative humidity must be kept low at the beginning (70 percent) to avoid frost forming on the meat surface, with an increase at the end of the thawing period up to 90–95 percent. In these conditions thawing of beef carcasses lasts about four to five days and of smaller carcasses one to three days. It must take place in installations specifically designed for this purpose.
Offal is not particularly influenced by the manner of thawing, but it is advisable to follow the same method.
Thawing is considered finished when the temperature of the meat is about 0° to -1°C.
When frozen meats are further processed they may in some cases be used directly in the frozen state. The consumer can start cooking small prepackaged cuts without prior thawing.
New quick thawing systems that satisfy hygienic requirements are now being used in the meat industry. Rapid thawing tunnels for carcasses, microwave ovens and tunnels and vacuum steam-heated autoclaves are some of the novelties. Thawed meats deteriorate quickly and must be kept at about 0°C and consumed as soon as possible.
Obviously a badly conducted freezing operation and/or frozen storage period (which includes transport and distribution) with irregular storage conditions will produce disorders in meat which become immediately apparent after thawing. Exudation indicates histological damage by ice crystals; other undesirable changes have already been mentioned.